Polymeric coatings which are used in packaging various material may comprise polymeric films. In many instances, the films comprise polyacrylates prepared by radiation curing of acrylate monomer and oligomers. However, there are certain disadvantages present when using this type of coating, especially when the coating may come in contact with edible materials. This is due to the fact that portions of the uncured acrylate material may leach out of the coating or film and be absorbed by the edible material. Therefore, it is necessary to provide some new type of coating or film, especially for materials which will be used in aseptic food packaging. In addition, another disadvantage which is present when utilizing uncured acrylate material in industry is a possible sensitivity of workers to the material thereby involving potential disruption of work due to health problems of the workers.
Generally speaking, it is desirable in the coating industry to use materials which, in their natural state, are in the form of free flowing liquids which may then undergo a rapid phase change to a solid state. The phase change which is undergone by the monomeric or polymeric material should occur at a rate which is commensurate with the processing steps which are contiguous to the process. The coating industry may utilize various methods to effect or produce the desired phase change. For example, the conversion of various compounds from the liquid state to the solid state as exemplified by paints, inks, coatings, etc. will undergo a phase change as the volatile solvent which is employed to provide the liquid phase is driven off. Alternatively, another method of effecting the desired phase change may be used when employing a two-component system in which a chemical reaction between the components causes or effects the change in the phases. In addition, there are also systems where a phase change is effected as a result of a polymerization reaction or a cross-linking reaction to produce a material which possesses a higher molecular weight than the monomeric or oligomeric compound.
The polymerization of a monomer or a polymer precursor may occur by a thermoactivation of the compound or it may occur by the application of radiant energy to the monomer or oligomer compound. When utilizing the latter approach to polymerization, two types of radiation are most commonly used, said radiation involving ultraviolet light or electron beam energy. Although both types of radiation may be used to induce a polymerization reaction, the types are quite different in nature. Ultraviolet light is selectively absorbed by molecules which possess appropriate chromophores. Wave lengths which are generally applied will range from about 500 to about 200 nm (57 to 143 Kcal/mol). This range will provide sufficient energy to place a molecule in an electronically excited state; however, it does not provide enough energy to directly ionize most molecules. The electronically excited molecules may react to produce a species which is capable of initiating a polymerization reaction, or they may decay back to the ground state.
In contradistinction to this, electron beams comprise streams of charged particles which have been generated by a cathode ray gun. The electrons will usually possess given energies ranging from about 100 to about 500 Kev (2300 to 1150 Kcal/mol). This energy is more than sufficient to easily ionize any organic material. The thus-ionized species may then take part in a polymerization reaction.
Most of the commercial coatings which have been cured by the application of ultraviolet light contain a photoinitiator along with monomers and/or oligomers which possess terminal acrylate functional groups. The photoinitiator is present to absorb the light and thereafter react to form a radical species This radical species initiates a rapid long-chain polymerization with the acrylic double bonds to produce the polymeric film which comprises the cured coating.
In addition, almost all of the commercial systems which are curable by use of an electron beam also contain acrylated monomers and oligomers. However, it is not necessary to utilize photoinitiators in these acrylate systems inasmuch as electron beams afford sufficient energy to directly ionize the acrylates. As also hereinbefore set forth, the acrylates comprise extremely useful materials for radiation cured systems. They possess cure speeds which are comparable to other process steps and in addition, a wide range of acrylated monomers and oligomers are commercially available in sufficient quantity to enable manufacturers to produce many different types of radiation cured products.
While acrylates are cured by free radical polymerization, it is also possible to effect the desired result by utilizing a cure system which comprises cationic polymerization. In U.S. Pat. No. 3,708,296, this type of polymerization was described wherein aryldiazonium salts were decomposed by ultraviolet light to produce a Lewis acid. The thus produced Lewis acid would then catalize the cationic polymerization of epoxy funtionalized monomers and oligomers. However, a major disadvantage which is present when utilizing the diazonium salts lies in the fact that these compounds are relatively unstable by nature and thus result in a product which has limited shelf life. U.S. Pat. Nos. 3,721,616, 3,711,390 and 3,711,391 describe various additives which may be added to the formulation in a attempt to stabilize the systems. However, in spite of the use of the gelation inhibitors, the mixtures still possessed an instability which made them generally unacceptable for commercial coatings.
U.S. Pat. Nos. 4,058,400 and 4,058,401 speak to the use of certain aromatic onium salts of Group VI A elements which may be used to catalyze a cationic polymerization reaction utilizing ultraviolet light irradiation as an energy source. The coatings which were prepared when using sulfonium salt initiators possessed significantly greater stability when compared to similar compositions which had been prepared but which had used a diazonium salt as an initiator. The latter patent speaks mainly to the polymerization of epoxy compounds while the former speaks to other cationically polymerizable materials which include multifunctional vinyl ether monomers.
Other U.S. Patents also disclose radiation induced cationic polymerization. For example, U.S. Pat. No. 4,069,054 describes the photosensitized decomposition of aromatic sulfonium salts. The use of such salts improves the spectral response of the system and thus allows irradiation at longer ultraviolet wave lengths. U.S. Pat. No. 4,069,056 also describes the use of Group V A aromatic onium salts as radiation sensitive catalysts for cationic polymerization. U.S. Pat. No. 4,090,936 describes a hybrid photohardenable composition containing mixtures of epoxides which have been cured by a cationic polymerization and acrylates which have been cured by a radical polymerization reaction.
Another U.S. Pat. No., namely 4,173,476, describes an initiator comprising an aromatic sulfonium salt which possesses a structure different in nature than those previously used. It is asserted in this patent that the use of such a photoinitiator will result in the formation of thick films, that is, up to about 1/2" in thickness resulting from polymerization of epoxide compounds. U.S. Pat. No. 4,250,203 utilizes the presence of an organic sulfur compound scavenger in cationic polymerization reactions to reduce or eliminate the sulfur odor which is generated upon the photodecomposition of the sulfonium salt photoinitiator. U.S. Pat. No. 4,264,703 also describes the use of aromatic iodonium salts as radiation sensitive catalysts for cationic polymerization.
As will hereinafter be set forth in greater detail, it has now been discovered that a semi-interpenetrating polymer network composition may be prepared by treating a mixture of a vinyl ether and a cellulose ester in the presence of a polymerization catalyst comprising an onium salt by treating the mixture with irradiation emanating from an electron beam source. The resulting composition will find a wide variety of uses of the type hereinafter set forth in greater detail.